1. Field of the Invention
The present invention relates to a milling and classifying apparatus that is utilized to prepare toners for electrostatic images from coarse toner particles by use of high-pressure and high-velocity air stream; and a method for producing a toner, the resulting toner, a collision mill, an air classifier, and an apparatus and a method for producing fine particles.
2. Description of the Related Art
Toners are typically utilized for developing electrostatic latent images in image forming processes such as electrophotographic processes and electrostatic photography processes. In theses processes, toners are demanded to be fine particles, and are typically produced by way of melting and kneading a binder resin, a colorant agent such as dye and pigment, and a magnetic material to prepare a mixture, then cooling-solidifying, and milling-classifying the mixture.
With respect to processes for producing toner fine particles from toner coarse particles by means of collision mills in the prior art, Japanese Patent (JP-B) No. 3133100 discloses a secondary collision plate, which is mounted to a grinding room, and is detachable in relation to the velocity of jet stream; JP-B No. 3090558 discloses a jet mill in which the inner surface of the grinding room has the same solid angle with that of the outer surface of the conical member to which the coarse particles are clashed and milled; Japanese Patent Application Laid-Open (JP-A) No. 08-103685 discloses a jet mill that is equipped with an inner wall of milling room where the third grinding is performed after the second grinding; and Japanese Utility Model Application Publication (JP-Y) No. 07-25227 discloses a jet mill in which the surface of the collision plate is flat and perpendicular to the axis of the nozzle, and a conical projection is disposed on the collision plate and is aligned with the axis of the nozzle.
FIG. 1 shows a typical construction of conventional jet mills. As shown in FIG. 1, coarse toner particles A to be milled are fed from inlet 13 of collision mill 11 into injection nozzle 12. High pressure air B is fed into injection nozzle 12, thereby the coarse toner particles flow with the stream of the high pressure air under higher velocities, then collide with collision plate 15 and are milled into finer particles. The milled toner particles C travel between the support 16 of collision plate 15 and the inner wall of grinding room 14, then flow out from outlet 17.
Recently, there exist commercial needs to improve dot reproducibility for higher image quality and to enhance fixing property at lower temperatures for energy saving, thus the toners are demanded to be more fine in their particle size and more narrow in their particle size distribution. As for additive materials compounded into toners, resins with lower softening temperatures are employed that have lower softening temperatures, and waxes are also added so as to agree with oil-less apparatuses. Consequently, there arise problems that the toners are hardly milled into desired particle sizes and various adhesion and/or deposition tend to generate in the production and/or processing facilities.
However, the proposals on the base of milling and classifying apparatuses described above address either reducing the particle size or narrowing the particle size distribution, in general. Therefore, both of reducing the particle size and narrowing the particle size distribution inevitably lead to reduction of toner feed rate currently, which resulting in lower productivity and higher production cost.
In conventional processes, toner particles milled by means of collision mills are further classified in order to remove coarse particles as well as excessively fine particles, thereby toner particles are prepared with an intended particle size distribution. FIG. 2 exemplarily shows a conventional air classifier that utilizes pressurized gas and high velocity stream (e.g. JP-A No. 2002-143775).
As shown in FIG. 2, since the lower surface of center core 28 in classification room 24 presents substantially the same slope with that of the upper surface of separator core 26, the powder of toner particles tends to flow stably between the lower and upper surfaces as a circular path. Namely, the fine particles balanced for the centrifugal force and the centripetal force may reside at certain sites of the lower surface of center core 28 and the upper surface of separator core 26 while swirling on the circular path, thus such balanced particles tend to enlarge the apparent size due to coagulation with other particles, consequently coarse particles are likely to yield. When such coarse particles are present in a toner of final product, the toner cannot represent a narrow particle size distribution, and also the coarse particles are likely to be divided into extremely fine particles in the preceding processes, which often degrading image quality remarkably.
Moreover, such balanced particles have a tendency to deposit on the lower surface of center core 28 and/or on the upper surface of separator core 26, which may affect the optimum classifying condition due to the deformation of classification room 24. Incidentally, less output rate of toner particles may bring about a narrower particle size distributions in precise classifying processes owing to less coagulation of fine particles; however, the decreased output rate inevitably leads to raising the production cost.
JP-A No. 07-155697 discloses an air classifier base on Coanda effect, in which the classification accuracy is enhanced by way of a rounded outer edge of a center core in a classification room. JP-A No. 06-154708 discloses an air classifier for the purpose of enhancing the classification accuracy, in which a separator core is divided into a central portion and an outer guide and a certain space is provided between them, thereby free vortexes generate within the classification room. JP-A No. 05-34977 discloses a method of producing a toner for the purpose of enhancing the classification accuracy, in which a guide room is provided above the classification room, plural louvers are provided between the guide room and the classification room, and powder and air fed into the guide room flow into the classification room through between the louvers. JP-A No. 2000-157933 discloses a classifier for the purpose of enhancing the classification accuracy, in which a kinetic energy is applied to a powder through controlling the air stream within a dispersion room, thereby the powder is sufficiently dispersed within the dispersion room.
However, these proposals in the prior art are insufficient for satisfying production capacity as well as classification accuracy in terms of the requirements for toners that are utilized in developing electrostatic latent images currently. For example, in the classifier of JP-A No. 2000-157933 described above, the air stabilizer in the dispersion room tends to decrease the stabilization effect with time, since the divided particles have a tendency to coagulate again till the particles flow into the classification room at downstream.